Tubular head exchangers are commonly found in large boiler systems and are typically used to increase operational efficiency or as part of the pollution process. Where the heat exchanger is used to increase operational efficiency, the heat exchanger converts thermal energy present in waste gas generated in combustion for use in another process. For example, in the electric utility industry, the combustion of coal generates a significant volume of hot flue gas. Following combustion, the flue gas exits the boiler outlet duct and travels into a tubular heat exchanger. At the tubular heat exchanger, the heat and energy contained in the flue gas is transferred into a plurality of thinly walled metal tubes. Large fans are used to push ambient air over the thinly walled heated tubes and an exit duct carries the air to the boiler for combustion. This process raises the temperature of combustion air entering the boiler, thereby improving boiler thermal efficiency and reducing net fuel consumption.
Outside of improving operational efficiency, tubular heat exchangers serve an integral role in the pollution control process. The vast majority of pollution control systems are designed around strict velocity, temperature, and volumetric parameters. For example, a fabric filter baghouse is an air pollution control equipment device that is used to collect particulate matter present in flue gas streams to prevent the emission of the fine particulates into the atmosphere. The efficiency and longevity of the fabric filter baghouse depends upon the operator's ability to control the temperature and velocity of the flue gas as it enters the fabric filter baghouses. In the Portland cement industry, a tubular heat exchanger is used for the sole purpose of controlling flue gas temperature and velocity prior to particulate matter collection at the fabric filter baghouse.
With respect to design, tubular heat exchangers are fixed structures consisting of a plurality of thinly walled straight metal tubes. Groups of tubes, often referred to as bundles or banks, extend through the outlet ducting of the boiler and open at the end of the ducting. The tubes are mechanically rolled or expanded into a thick metal tubesheet and intermediate baffles may be used to provide structural support. The entire system is enclosed in a steel casing which serves as the enclosure for the air or gas passing outside of the tubes.
Tubular heat exchangers are most commonly classified as either vertical or horizontal designs. In the vertical design, tube bundles are arranged vertically and flue gas enters through the inner diameter of the heat exchanger tubing. At the top of the system, the tubes are mechanically rolled or expanded into tubesheet plates located at the top and bottom of the structure. The tubesheet is made from steel that ranges in thickness from one inch to three inches. The tubesheet plate provides physical stability for the structure and also creates a seal between the air and the gas flow.
In a horizontal design, the tube bundles are arranged horizontally. Similar to the vertical design, the horizontal tubes are mechanically rolled or expanded into the tube sheet plates. The primary distinction between the horizontal and vertical designs is that hot flue gas travels over the outer diameter of the tubes with combustion air passing through the inner diameter of the tube in the horizontal design.
Common in both the horizontal and the vertical designs is the use of anti-vibration baffles. The anti-vibration baffles provide structural support for the tube length and also help prevent against the occurrence of mechanical damage that may result from the vibration of tubing while the boiler is in operation. The anti-vibration baffles typically comprise steel plates containing ‘baffle holes’ that allow the tubes to pass through the plate. The anti-vibration baffle levels will be arranged either horizontally or vertically, perpendicular to the tubes, and multiple baffle levels may exist depending on the tube length and design specifications. The baffle location, or level, may exist at various lengths but are most commonly found at approximately four to six feet away from the nearest tubesheet plate or next anti-vibration baffle.
Additionally, pass partition plates are used to redirect air and/or gas flow in order to force the air and/or gas to make multiple passes over the tube bundles. The pass partition plates appear substantially similar to the anti-vibration baffles, with tubes passing through holes contained in the pass partition plates. The primary distinction of the pass partition plate is that the length of the pass partition plate extends further than the anti-vibration baffle to the casing wall that encloses the heat exchanger.
During boiler operation, fly ash generated during combustion commonly accumulates at the anti-vibration baffles, pass partition plates, or at the bottom of the tubesheet plates. The volume and chemical composition of the fly ash varies greatly. Factors that may impact the fly ash composition include fuel source, pollution control method systems, and boiler use. The volume of fly ash at various tubular heat exchanger levels may range from minimal to over several feet in extreme cases.
During outages, it is common practice to clean or remove the fly ash that has accumulated on the equipment used in the boiler system. In tubular heat exchangers, one cleaning method includes using high pressure water to push the fly ash from the tube length, anti-vibration baffle, pass partition plate, or the top and/or bottom tubesheets. The fly ash is then ordinarily collected in an ash hopper located beneath the heat exchanger.
This cleaning method commonly results in the accumulation of both water and fly ash in the tubular heat exchanger. This frequently results in the creation of a highly acidic slurry at the tubesheet plates, anti-vibration baffles, and pass partition plates. The acidic slurry may remain stagnant during the length of the outage and present during portions of operation.
Tube bundles remain in continuous contact with the acidic slurry at the baffle or plate level. At the baffle level, the slurry also drips downward and onto the tube length. This does not occur at the tubesheet plate level where the tubes have been rolled into the plate.
Contact with the acidic slurry causes the tubing to corrode from the outside in. This damage mechanism is referred to as ‘external corrosion’ and commonly results in tube failures.
A heat exchanger tube is considered to have failed where a hole or leak has developed on the tube surface. The hole allows air or gas to leak into the stream and the failures substantially decrease the efficiency of the heat exchanger and cause a decrease in the boiler's overall efficiency.
Where boiler efficiency has been decreased, negative results include increased fuel consumption, increased rates of internal power consumption, and decreased performance of pollution control systems. A tube that has developed a failure requires replacement or repair.